SBC Cheatsheet:
Far End NAT Traversal (FENT)

When deploying Remote Phones, whether in a Hosted Solution or with Home or Remote workers, these phones will reside behind some type of firewall. Firewalls are meant to provide security by hiding the private network addresses and prevent all unsolicited network traffic from entering the private network. In addition, the SIP Protocol (RFC 3261) is an application and many firewalls don’t provide application support.